Dispenser cathode material and method of manufacture



Jan 6, 1970 L. H. AWA- 3,488,549

DISPENSER CATHODE.MATERIAL AND METHOD OF MANUFACTURE Filed Jan. 15, 1968WELD TI; bw/ 0m Iva-45:: orPe p1 4V-W*Z5Pe Invervtor: LutFi Amra b3 Wilt.Hs A ir t'ovheg United States Patent 0 US. Cl. 313346 16 ClaimsABSTRACT OF THE DISCLOSURE Dispenser cathode material is provided in theform of wire with thorium-bearing emission material at the center and asheath of tungsten or tungsten-base alloys, particularly tungsten-25% byweight rhenium, and with a barrier of molybdenum or rhenium between thetungsten alloy sheath and the emission material. The barrier preventscontamination of the sheath by thorium and may prolong life of thecathode by preventing or impeding thorium attack of the tungsten. Aprocess is provided for producing such dispenser cathode material bymechanically deforming and elongating a billet which comprises atungsten-rhenium sheath in cylindrical form with one end closed, insidethe sheath a molybdenum or rhenium tube of substantial thickness, andthe tube filled with the emission material. By preventing thorium grainboundary corrosion of the sheath, the barrier keeps the sheath intactand permits deformation of the billet. Processes of canning the billetin molybdenum, and extruding, swaging, and continuously drawing it downto the desired wire size are quite economical in producing dispensercathode material which can be cut into short lengths and one end ofwhich can be capped.

The cathode material of the invention is particularly useful inproducing nearly circular cathodes of uniform and reproducible highquality. Such cathodes are capable of emitting electron beams of verysmall diameter, such as might be used in producing diffraction gratingson deformable light-valve media employed for the production ofthree-color television pictures.

BACKGROUND OF THE INVENTION This invention relates to dispenser cathodematerial and a method for making the same. More particularly, it relatesto such material in wire form and individual dispenser cathodes cut fromsaid wire.

Dispenser cathodes for vacuum tubes and the like containing a quantityof electropositive material have the advantage of long life and highelectron emission density. In the usual dispenser cathode, applied heatcauses the electropositive material to be dispensed from the cathodeinterior through a porous barrier, forming a monatomic layer upon thecathode surface at a rate substantially equal to the evaporation of suchmaterial from the surface. Cathodes of this type are employed in highpower transmitting tubes and the like, wherein they present a relativelylarge emitting surface area and produce a rela tively broad and somewhatscattered electron beam. This emission usually takes place not only fromthe designated 3,488,549 Patented Jan. 6, 1970 emitting area, but alsoas edge or side emission from the cathode body around the emitting area,the breadth of the emitted beam frequently depending upon the cathodesize. For producing a smaller concentrated electron beam, a smallcathode would be desirable; however, dispenser cathodes conventionallyrequire bulky heating coils for elevating the cathode to the properemitting temperature, placing a lower limit on the size of the cathode.

Small-dimension electron beams are ordinarily produced with the morecommon type of directly heated or indirectly heated cathode. Theconventional cathode ray tube, for example, includes an indirectlyheated cathode at the rearward extremity of the electron gun forproducing a narrow stream of electrons. The beam produced in this typeof cathode ray tube is suitable for wave-form portrayal, televisionreproduction and the like. However, in many applications an electronbeam of yet smaller dimension is required. A small electron beam isrequired, for example, for writing finely detailed charge patterns on acharge receiving surface. The charge pattern may comprise, for example,the lines of a three-color diffraction grating on a deformablelight-valve medium employed for the production of three-color televisionpictures. An apparatus employing such charge pattern is illustrated andclaimed in Patent 2,813,1l46 to William E. Glenn, Jr., issued Nov. 12,1957, and assigned to the assignee of the present invention.

Suitable dispenser cathodes for producing such fine electron beams aredescribed and claimed along with a method for their manufacture inPatent 3,263,115-Glascock et a1., issued July 26, 1966, and assigned tothe assignee of the present invention. That patent describes a processwherein individual cathodes are manufactured by plugging one end of finediameter tantalum tubing with pressed tantalum powder and sintering tostrengthen the powder, filling the tube with emission material such asthorium metal or barium-aluminum alloy, and sealing the other end. Thepatent states that tungsten sheaths are preferable for such cathodeswhich are to be used at higher temperatures, although their emission isnot as good at lower temperatures as cathodes made with tantalumsheaths. Due to the unavailability of small tungsten tubing, whentungsten has been used it has been necessary to use expensive techniquesto produce a cup and fill it with emission mix. The cathodes wereproduced by starting with an 0.050 inch length of 0.030 inch diametertungsten wire and machining a hole 0.040 inch deep by 0.017 inchdiameter into the center of the wire from one end. Electrical dischargemachining involving costly equipment and expensive set-up operationsseems necessary for this machining. The emission powder mixture was thenadded a mil at a time and pressed down into the hole by a plunger.

It would be desirable to have a process available for producingtungstenand tungsten alloy-sheathed small diameter dispenser cathodessuitable for forming fine electron beams by techniques that would notrequire the major expenses of individual handling, machining, fillingand compacting of the tungsten cathodes of the prior art.

Previous attempts to produce such cathode materials in continuous formby compacting a core of thoriumcontaining emission materials in atungsten or tungstenbase alloy sheath to make a billet and deforming thebillet to produce a wire, have been unsuccessful. This is primarilybecause of attack by the thorium metal on the tungsten sheath materialduring sintering and deformation and accompanying anneals, and probablycontinuing during operation of the resultant cathodes.

SUMMARY OF THE INVENTION Thus, it is an object of the invention toprovide tungsten-sheathed, thorium-containing, small diameter dispensercathode wire material in continuous form at low expense and in areliable form which can be used readily. A further object is to providea process suitable for the production of such cathode materials.Additionally, another object of the invention is to provide such cathodematerials and processes for their manufacture in which the thorium willnot diffuse destruetively into the tungsten sheath.

Brieffy stated, the present invention in certain of its embodimentsprovides dispenser cathode material in wire form having an outer sheathof tungsten or alloys of tungsten, preferably tungsten containing 5 to35% rhenium, or more preferably tungsten with about 25% rhenium, with acore of emission material including thorium, and preferably containing,in powder form, thorium and tungsten carbide, or tungsten, thorium, andtungsten carbide, with optimum proportions being about 6 3% W, 20% Th,and 17% WC. Percentages herein are by weight. A tungsten-base alloy isherein defined as an alloy containing at least 50% tungsten. A barrierof molybdenum or rhenium of substantial thickness, provided between theemission material and the tungsten sheath, acts as a diffusion sink orbarrier for thorium and prevents further deleterious grain boundarydiffusion of the thorium out into the tungsten which could lead todestruction of the cathode body, or emission from the side of thecathode which would impair the desired small diameter electron beamproduced by the cathode. Although pure molybdenum or rhenium isgenerally preferred, essentially equivalent molybdenum-base orrhenium-base alloys can be used. Individual cathodes can be cut fromsuch material and preferably have one end sealed and a length not morethan a few times the wire diameter.

The method of production of the dispenser cathode material according tothis invention comprises providing the tungsten sheath in the form of acylinder with one end closed of tungsten or tungsten alloys, preferablywith 25% rhenium, fitting into this sheath a molybdenum or rhenium tubehaving substantial thickness, and filling the tube with compactedemission material. The open end of the sheath preferably is sealed over,such as by welding a sheet of molybdenum onto the end, to form a billet.The billet is then mechanically deformed to produce the wire. Preferableprocesses include canning the billet in molybdenum for oxidationprotection and lubrication, and extruding it at elevated temperatures ata sutfieient ratio of reduction in cross section to permit furtherworking as by swaging, generally at least 3:1 or 4:1, preferably 10:1,dissolving the can, and then swaging and drawing down to the desiredwire size. Recrystallization anneals after about every 40% of reductionin area are quite desirable if not necessary for producing the dispensercathode with a sheath of the preferred tungsten- 2.5% rhenium alloy.Fewer anneals would be necessary if the sheath were pure tungsten. Incertain embodiments of the invention, it is desirable to insert one ormore layers of tungsten or molybdenum or rhenium foil such as of about0.015 inch thickness inside the tungstenrhenium cylinder before fittingthe molybdenum or rhenium tube into the cylinder. This foil serves as anadditional impediment to diffusion by creating an extra interface whichcan persist during sintering and annealing processes.

4 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional schematicelevation of a billet for producing certain dispenser cathode materialsby processes of the invention.

FIG. 2 is an elevation view partly in section of a dispenser cathode ofthe invention ready for use.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Dispenser cathode wire materialhas been fabricated according to the invention to small wire 0.030 inchin diameter with an essentially concentric core of emission material,and it can readily be produced in smaller sizes if desired. Cathode gunshave been produced with lengths of this wire 0.030 inch in diameter by0.05 inch long and their emission characteristics and lifetimesevaluated at high voltages under simulated actual use conditions.

This cathode material was produced by a process described below,exemplary of the invention.

A powder mixture of W-25% Re was pressed into a billet 1.25 inches indiameter by 2.5 inches long. The billet was presintered at 1200 C. fortwo hours to increase its strength. A hole 0.430 inch in diameter by 2inches deep was then easily drilled in the billet. Sintering of thealloy was done at 2750 C. for two hours in order to get density over oftheoretical. Then, the inside walls of the cylinder were lined with0.015 inch thick tungsten foil, and a molybdenum tube with 0.075 inchthick walls was fitted into the cavity. It is preferable but notnecessary to use a molybdenum tube that has one end closed. Themolybdenum tube was then filled with powder consisting of 63% tungsten,20% thorium, and 17% tungsten carbide, and this powder was denselypacked in the tube by tapping the billet. Final sintering wasaccomplished at 1550 C. for two hours in vacuum at a pressure lower than10- atmospheres with some reaction of the thorium reducing the tungstencarbide to produce thorium carbide. The rate of heating was very slow inorder to insure purification, particularly of the emission mix powder.The heating schedule was as follows:

1 hour to 1100 C.,

1 /2 hours at 1100" C., 1 hour at C., and 2 hours at 1550 C.

The sintered billet was jacketed in molybdenum, which acted as alubricant and gave some oxidation protection during extrusion. This gavea jacketed billet in accordance with FIG. 1. Extrusion was accomplishedon a Dynapack machine with the billet at 2050 C. and using a 10.421reduction ratio. The firing pressure for extrusion was 1200 pounds persquare inch, equivalent to 312,000 inch-pounds of energy. The extrusionrod Was 0.35 inch in diameter by 12 inches long.

Before commencing swaging, the molybdenum jacket was removed by etchingor dissolving it in a solution of one part HNO one part H 50 and threeparts H O by volume. Swaging started at 1650 C. with a gradual loweringof the temperature to 1300 C. on further swaging. Because W-25% Re alloyis known for its rapid work hardening, it was considered necessary notto exceed about 40% reduction in area between anneals. Initialrecrystallization anneals were accomplished at 2000 C. for five minutes.Upon further swaging, the recrystallization temperature was lowered to1600 C. for fifteen minutes.

Wire drawing was carried on at a much lower temperature. Because of thehigh ductility of W-25% Re, it was feasible to draw the wire at sizesbelow 0.044 inch diameter at room temperature. A summary of the swagingand drawing reductions and temperatures and the recrystallizationanneals (Rexl) appears in the table below.

TABLE.SWAGING AND DRAWING SCHEDULE OF CATHODE EMISSION MATERIAL From,To, Percent Temp., 0. inches inches R.A.

S a in z w nit ial 0. 330 0.285 27 2,020, minutes 0. 285 0.243 27 2,020,5 minutes 0. 243 0. 200 2,200, 5 minutes 50 0. 200 0. 163 2,020, 5minutes ,3 50 0.163 0. 134 31 1,600,15 minutes 1,300150 0.134 0.105 381,600, minutes- 1,3 i50.-.- 0.105 0.082 40 1,600, 15 minutes 1,3 ;l;500.082 0. 064 40 1,600, 15 minutes. T3... 1,1001100 0. 064 0.050 391,600, 15 minutes 00i100 0.050 0.044 22 x 1,600 15 minutes 900a; Rm.Temp. 0. 044 0. 030 46 To make the composite wire product more ductileand easier to handle in subsequent fabrication, a final anneal at thefinished size, in this case 0.030 inch diameter, is desirable. The wirewas treated in a hydrogen atmosphere at temperatures between 1200 and1600 C. for 15 minutes. Anneals at temperatures between 1200 and 1500"C. resulted in substantial decreases in the hardness of the materialfrom the initial hardness of 640 DPH down to the range of 560600 DPH andthe treatment at 1600 C. resulted in a major decrease in hardness downto about 450 DPH. Based on these heat treatments, the optimumtemperature to ductilize the wire is around 1500 to 1600 C.

Wire produced according to this procedure is continuous with asubstantially concentric core of emission material. The thorium did notdiffuse out into the sheath and cause intergranular corrosion as didoccur in previous experiments in which the wire was produced withoutusing the molybdenum barrier layer. Diffusion of the thorium seems tohave been arrested by the molybdenum acting as a diifusion sink. Thetungsten foil with its additional interface between the molybdenum andthe tungsten sheath likely contributed to minimizing dilfusion.

When rhenium is used instead of molybdenum as the barrier material, morefrequent anneals will be required as is known in the art due to workhardening of the rhenium. The mechanism of reaction of thorium withtungsten and its alloys seems to be diiferent than with molybdenum andrhenium. Harmful grain boundary attack of thorium on tungsten-25%rhenium has been noted. There is limited general solubility of thoriumin molybdenum, and grain boundary attack has not been observed. Thismechanism appears to prevent destruction of the molybdenum barriercathode by thorium corrosion. Rhenium does not seem to be attacked bythorium either along its grain boundaries or by general dissolution, butprovides a good barrier to any thorium penetration.

Cathode wire material with a molybdenum barrier is less expensive due tolower material and working costs, and is satisfactory for certainapplications. The material made with a rhenium barrier is more expensivebut can have a longer life in vacuum applications, since the thorium andmolybdenum may tend to evaporate together over a period of time.

Although the cathodes described above were produced with emission mixescontaining W, Th, and WC in the stated proportions, it is also feasibleto use emission mixes of about 50% Th, 50 WC, which possess a largerthorium supply, but this fill material is less structurally strong thanthe one also containing WC.

Thermal life tests on emitters produced in the configuration of FIG. 2from dispenser cathode elongated wire material of the invention byclosing one end of a short length of the wire produced as described indetail above with a tungsten weld have shown satisfactory results. Thesetests involve holding the emitter at a temperature around 1600 C. for anextended length of time, and periodically measuring emission pulses todetermine the electron emission efficiency. Some such tests withemitters of the invention have demonstrated a useful pulse life inexcess of 6000 hours.

The foregoing is a description of illustrative embodiments of theinvention, and it is applicants intention in the appended claims tocover all forms which fall within the scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Dispenser cathode material in wire form comprising thorium-bearingemission material at the center of the wire laterally enclosed by asheath of metal selected from the group consisting of tungsten andtungsten-base alloys, said emission material and said sheath beingseparated by a continuous layer of a metal selected from the groupconsisting of molybdenum and rhenium.

2. Dispenser cathode material according to claim 1 in which saidemission material contains thorium, tungsten, and carbon.

3. Dispenser cathode material according to claim 2 in which saidemission material consists essentially of tungsten, thorium, andtungsten carbide, in which sulficient tungsten carbide is present toconvert substantially all of said thorium to thorium carbide.

4. Dispenser cathode material according to claim 3 in which said thoriumhas been converted to thorium carbide.

5. Dispenser cathode material according to claim 1 in which said sheathis a tungsten-base alloy containing rhenium.

6. Dispenser cathode material according to claim 5 in which said alloyconsists essentially of tungsten with about 25% by weight of rhenium.

7. Dispenser cathode material according to claim 1 which has been cutinto short lengths, with their lengths being not more than a few timesthe wire diameter, and one end of each of said lengths having beensealed with a metal selected from the group consisting of tungsten andtungsten-based alloys.

8. A process for producing dispenser cathode material of claim 1 inwhich a billet is produced comprising a cylindrical sheath with one endclosed, said sheath being composed of a metal selected from the groupconsisting of tungsten and tungsten-based alloys, a tube of materialselected from the group consisting of molybdenum and rhenium ofsubstantial thickness fitted into said sheath, and thorium-containingemission material in said molybdenum tube, the open end of said billetbeing sealed, and

said billet is mechanically deformed to lengthen it and reduce itsdiameter to produce said elongated dispenser cathode in wire form.

9. A process of claim 8 in which a layer of tungsten foil is providedbetween said sheath and said bolybdenum tube.

10. A process according to claim 8 in which said sheath is composed of atungsten-based alloy containing rhenium.

11. A process according to claim 8 in which the sheath is composed of analloy consisting essentially of tungsten with about 25% by weightrhenium.

12. A process according to claim 8 in which the emission materialcontains thorium, tungsten and carbon.

13. A process according to claim 8 in which the emission materialconsists essentially of thorium, tungsten and tungsten carbide in whichsufficient tungsten carbide is present to convert substantially all ofsaid thorium to thorium carbide.

14. A process according to claim 8 in which said emission materialconsists essentially by weight of about 63% tungsten, 20% thorium, and17% tungsten carbide, all mixed together in the form of a powder.

15. A process according to claim 14 in which said emission material isexposed to elevated temperatures for a time sufiicient to convertessentially all the thorium to thorium carbide.

16. A process according to claim 8 in which the sheath consistsessentially of an alloy of tungsten with about 25% by weight rhenium,

the emission material is a mixture of powders of tungsten, thorium, andtungsten carbide,

the billet is enclosed in a molybdenum jacket,

the jacketed billet is extruded for a substantial reduction incross-sectional area and said jacket is then removed from said billet,and

the extruded billet is then further deformed by swaging followed bydrawing to the ultimate desired size, With said extruded billet beingrecrystallized by heat treatments after reduction in area of not morethan about before further deformation.

References Cited UNITED STATES PATENTS 3,232,717 2/1966 Hill et al.313-346 X 3,436,584 4/1969 Hughes 313-346 X FOREIGN PATENTS 1,186,953 2/1965 Germany. 1,046,639 10/ 1966 Great Britain.

JOHN W. HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner U.S.Cl. X.R.

